This invention relates to a photoreceptor, particularly to an electrophotographic photoreceptor
Inorganic photoconductive materials including selenium, zinc oxide, titanium oxide and cadmium sulfide have conventionally been used as light-sensitive materials in electrophotographic photoreceptors. Active R&D efforts have recently been made on amorphous silicon.
Organic photoconductive materials (OPC) are also usable as light-sensitive materials and increasing attention is being paid to OPCs since they are generally less toxic than inorganic photoconductive materials and since they are more advantageous in such aspects as flexibility, lightweightness, film-forming property and cost.
Whichever type of light-sensitive materials are used in electrophotographic photoreceptors, "functionally separated" photoreceptors in which different materials are used in two layers for fulfilling separate functions of charge generation and charge transport have the advantage that the two functions can be performed independently of each other, whereby greater latitude in photoreceptor design can be attained. Another advantage of "functionally separated photoreceptors" is that they have improved electrophotographic characteristics such as high sensitivity, long cycle life and good mechanical strength.
Such electrophotographic photoreceptors are extensively used in electrophotographic copiers, printers, etc. As recent models of electrophotographic copiers and printers are operated at faster speeds, the time required for copying processes is significantly shortened and at the same time, the number of copying cycles increases so much as to require the use of more sensitive and durable photoreceptors.
Polycyclic quinone compounds and azo pigments may be shown as typical examples of high-sensitivity carrier generating materials that are suitable for use in electrophotographic photoreceptors. White light sources such as an A light source and a fluorescent lamp are commonly employed with electrophotographic copiers and the photoreceptor is illuminated with light having wavelengths over the entire visible range. Polycyclic quinone compounds have excellent photocarrier generating ability but their spectral sensitivity is limited to the range of 400-570 nm. Hence, using polycyclic quinone compounds, it is difficult to fabricate electrophotographic photoreceptors having sensitivity to light at wavelengths longer than 570 nm. On the other hand, photoreceptors using azo pigments, for example, disazo pigments, have their spectral sensitivity extended to nearly 700 nm but their photosensitivity is low on the shorter wavelength side. Therefore, it is also difficult to fabricate photoreceptors with higher sensitivity using azo pigments.
A more important problem arises from the fact that electrophotographic copiers which are indispensable to everyday work in business offices, governmental agencies, public organizations, etc. are required to achieve faithful reproductions of original documents, particularly seals affixed to various documents and underlines marked in red. To meet this need for faithful reproduction of red images, the carrier generating materials used should not have any sensitivity in the wavelength range beyond 600 nm or they must have such a spectral sensitivity characteristic that the sensitivity in this longer wavelength range is lower than in the other ranges. However, the carrier generation materials available today have their own spectral sensitivity characteristics and it is considerably difficult to find carrier generating materials that have satisfactory electrophotographic characteristics and which yet are capable of reproducing red originals to a degree that suits the specific purpose of a copying operation.
With a view to improving the reproducibility of red color, it has been proposed that a red cutting cyan filter be inserted into the optical path of white light. However, if light at wavelengths longer than 600 nm is cut when disazo pigments are used, a substantial decrease in light sensitivity occurs.
As described above, prior art photoreceptors do not have sufficient light sensitivity and red color reproducibility to be adapted for use with high-speed electrophotographic copiers and small copiers and the advent of photoreceptors having higher sensitivity has been desired.